Discharge lamp which incorporates divalent cerium halide and cesium halide and a high mercury loading

ABSTRACT

ARC-DICHARGE DEVICE WHEREIN THE DISCHARGE-SUSTAINING FILLING INCLUDES AS ESSENTIAL ELEMENTS PREDETERMINED AMOUNTS OF AT LAST ONE OF PRASEODYMIUM HALIDE, NEODYMIUM HALIDE AND CERIUM HALIDE PLUS CESIUM HALIDE AND SUFFICIENT MERCURY TO PROVIDE AN OPERATING MERCURY-VAPOR PRESSURE OF FROM 3 TO 15 ATMOSPHERES, IN ADDITION TO THE USUAL STARTING GAS. THE RAREEARTH METAL HALIDE PROVIDES A VERY EFFICIENT DISCHARGE AND THE CESIUM HALIDE PLUS THE HIGH MERCURY LOADING PERMITS A HIGH EFFICIENCY TO BE OBTAINED WITH A RELATIVELY LOW MINIMUM ENVELOPE TEMPERATURE. THE RATIO OF TOTAL GRAM-ATOMS OF HALOGEN TO TOTAL GRAM-ATOMS OF METAL IN THE PRASEODYNIUM, NEODYMIUM OR CERIUM HALIDES IS FROM ABOUT 1.88:1 TO ABOUT 2.7:1, AND THIS PROVIDES A VERY DIFFUSE, STABLE DISCHARGE WHICH IMPROVES THE PERFORMANCES OF THE DEVICE. OTHER DISCHARGE-SUSTAINING MATERIALS DESIRABLE ARE ADDED TO THE FOREGOING ESSENTIAL COMSTITUENTS TO MODIFY THE COLOR OF THE DISCHARGE.

United States Patent [191 Zollweg et al.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

[22] Filed: July 21, 1972 [21] Appl. No.: 273,884

52 use! ..313/229,313/l84,3l3/225, Y Y 313/227 51 lnt.Cl. ..H01j61/20[58] Field of Search... 313/184, 225, 227, 228, 229 v PrimaryExaminer-Roy Lake Assistant Examiner-Darwin R. Hostetter AttorneypAgent,or Firm-A. T. Stratton et al.

[451 Mar. 19, 1974 5 7 ABSTRACT Arc-discharge device wherein thediscahrge-sustaining filling includes as essential elementspredetermined amounts of at least one of praseodymium halide, neodymiumhalide and cerium halide plus cesium halide and sufficient mercury toprovide an operating mercury-vapor pressure of from 3 to 15 atmospheres,in addition to the usual starting gas. The rare-earth metal halideprovides a very efficient discharge and the cesium halide plus the highmercury loading permits a high efficiency to be'obtained with arelatively low minimum envelope temperature. The ratio of totalgram-atoms of halogen to total gram-atoms of metal in the praseodymium,neo'dymium or cerium halides is from about 1.8:1 to about 2.7:], andthis provides a very diffuse, stable discharge which improves theperformance of the device. Other discharge-sustaining materialsdesirably are added to the foregoing essential constituents to modifythe color of the discharge.

9 Claims, 2 Drawing Figures 1 DISCHARGE LAMP WHICH INCORPORATES DIVALENTCERIUM HALIDE AND CESIUM HALIDE'AND A HIGH MERCURY LOADINGCROSS-REFERENCE TO RELATED APPLICATION The discharge device described inthis application is an improvement over the discharge device describedand claimed in copending application Ser. 243,711, filed- Apr 13, 1972,entitled Discharge Lamp Which Incorporates Cerium and Cesium Halides anda High Mercury Loading, by the present inventors, and owned by thepresent assignee.

BACKGROUND OF THE INVENTION This invention generally relates todischarge devices and, more particularly, to so-called metal halidedischarge devices where'in predetermined-amounts of selecteddischarge-sustaining materials are utilized in order to improve theperformance of the device.

In U.S. Pat. No. 3,234,421 dated Feb. 8, 1966, is dis-' closed aso-called metal halide discharge lamp wherein selected metallic halides,particularly those of Group IA, 11A, 11B, and IIIA are incorporated intothe device in order to modify the color of the discharge and theoperating efficiency of same with respect to the generation of visiblelight.

In U.S. Pat. No. 3,334,261 dated Aug. 1,- 1967, is disclosed ametal-halide discharge device which incorporates rare-earth 'metalhalides in order to modify the discharge; In U.S. Pat. No. 3,407,327dated Oct. 22, 1968, is disclosed a metal-halide discharge device whichincorporates afilling of alkali metal halide and scandium halide. InU.S. Pat. No. 3,514,659, dated May 26, 1970, is disclosed a metal halidelamp which incorporates arelatively small amount of cesium iodide inorder to reduce the so-called reignition voltage, with the cesium halidebeing limited in amount so that it will not lower the arc temperature toa point where the emission lines of the primary light emitting metalsare weakened. In U.S. Pat. No. 3,262,012 dated July 19, 1966 isdisclosed a discharge device which utilizes cesium and sodium halides inaddition to other rare-earth metal halidesnThe device is intended tooperate on a conventional ballast and the mercury loading which isutilizedis quitelow. Y

In U.S. Pat. No. 3,398,312 dated Aug. 20, 1968, is disclosed a metalvapor lamp which includes sodium iodide and a small amount of free metalwhich reacts with any free iodine released during operation of the lamp.

SUMMARY or THE INVENTION An arc-discharge device comprises a sealedelongated light-transmitting envelope which encloses a predeterminedvolume. As is conventional, electrical leadin conductors are sealedthrough the envelope and are electrically connected to electrodes whichare operatively spaced apart a predetermined distance within theenvelope. A discharge-sustaining filling is enclosed by the envelope andcontains the following as essential constituents: a small charge ofinert ionizable starting gas; mercury in predetermined amount asrequired, when fully vaporized, to produce an operating mercury vaporpressure of from 3to l5 atmospheres; at l ea t one of praseodymiuihhalide, neodymium halide and cerium halide, excluding the fluoride, intotal amount of from 2 X 10 to 2.5 X 10" gram mol/cm of spacing betweenthe lamp electrodes, and preferably from 1.4 X 10 to 5.4 X 10 grammol/cm of spacing between the lamp electrodes, with the ratio of totalgram-atoms of halogen to total gram-atoms of metal in the praseodymium,neodymium and cerium halides being from about 1.8:1 to about 2.7: l;cesium halide excluding the fluoride, in amount of from 3.5 X 10 to 2.5X 10 gram mol/cm of spacing between the lamp electrodes and preferablyfrom 3.5 X 10' to 5.4 X 10' gram mol/cm of spacing between the lampelectrodes; and the molar ratio of total praseodymium halide, neodymiumhalide and cerium halide to cesium halide is from 4/1 to 1/25. Otheradditives desirably are included with the discharge sustaining filling,such as sodium halide, dysprosium halide and samarium halide, with theseadditional added halides present in predetermined. amount.

BRIEF DESCRIPTION OF THE DRAWING ing only a sectional view of the innerarc tube which is formed of polycrystalline alumina or similarrefractory envelope material and which incorporates adischargesustaining filling in accordance with the present invention. I1

DESCRIPTION OF THE PREFERRED EMBODIMENTS With specific reference to theform of the invention illustrated in the drawing, the discharge deviceor lamp 10 is generally similar in construction to the usualhighpressure, mercury-vapor lamp and comprises a radiation-transmittingsealed inner envelope or arc tube 12 having electrodes 14 operativelydisposed proximate either end thereof and operable to sustain a vapordischarge therebetween. A charge of mercury 16 and a small charge ofinert ionizable starting gas such as 20 torrs of argon are containedwithin the inner envelope 12. The charge of mercury 16 is present inpredetermined amount as required, when fully vaporized as the soledischarge-sustaining constituent, to provide an operating mercury-vaporpressure of from 3 to 15 atmospheres as calculated on the basis of anaverage temperature for the vaporized mercury of 20001(. This averagetemperature may vary somewhat depending upon the variousdischarge-sustaining constituents which are used and the lamp operatingconditions, but this indicated figure is a representative averagetemperature for the vaporized mercury. Since the envelope volume isalways known, the required amount of mercury to provide the properoperating conditions can readily be calculated. During operation of thedevice, the other discharge-sustaining materials may interact with themercury to affect the actual operating pressure of the mercury and, inaddition, the extreme temperature gradients from the actual arc to theenvelope wall may have an effect on the actual pressure within theoperating device. For this reason, it is more accurate to express therequired amount of mercury as if that material, per

se, were the sole discharge-sustaining constituent, since the amounts ofmercury placed into the arc tube are known and the resulting pressure asdetermined by the foregoing mercury vapor temperature, can readily beascertained.

At least one of praseodymium halide, neodymium halide and cerium halide18, excluding the fluoride, is included within the arc tube 12 in totalamount of from 2 X 10 to 2.5 X 10" and preferably from 1.4 X 10 to .4 X10' gram mol/cm of spacing between the electrodes l4, and as a specificexample, the electrodes are spaced from each other by a distance of 7centimeters and the arc tube 12 encloses a volume of cubic cen timeters.Also included within the arc tube is cesium halide 20, excluding thefluoride, in amount of from 3.5 X 10' to 2.5 X 10 and preferably from3.5 X 10' to 5.4 X 10 gram mol/cm of spacing between the electrodes 14.The molar ratio of total praseodymium halide, neodymium halide andcerium halide to cesium halide is from 4/1 to l/2.5. In accordance withthe present invention, the ratio of total gram-atoms of halogen to totalgram-atoms of metal in the praseodymium halide, the neodymium halide andthe cerium halide is from about 1.821 to about 2.711. The normal stablevalence state for these metals is a plus three, but in the case of theindicated halides, the metals also can exhibit a valence state of plustwo. These metal halides can be dosed into the arc tube 12 either in thetwo-plus state or as a mixture of plus-two and plus-three states. Ratiosof halogen to metal of less than 2:] are obtained by dosing a smallamount of the rare-earth metal with the indicated rare-earth metalhalide. As a specific example, the atom ratio of halogen to rare-earthmetal is 2.5:1.

A radiation-transmitting, sealed outer envelope 24 is spaced from andsurrounds the arc tube and preferably the space between the arc tube 12and the outer envelope 24 is evacuated. Electrical lead-in conductors 26are sealed through both the inner arc tube 12 and the outer envelope 24and serve to electrically connect the operating electrodes 14 to aconventional power source (not shown).

A starting electrode 30 is also included within the arc tube 12 andconnects through a starting resistor 32 to one end of the electricallead-in conductors 26. The are tube 12 is maintained in specedrelationship from the outer envelope 24 by means of a conventionalsupporting frame 34. Ribbon conductors 36 serve to facilitatehermetically sealing the lead-in conductors through the ends of the arctube. The lead-in conductors are sealed through the outer envelope 24 bymeans of a conventional re-entrant stem press 38 and connect to astandard mogul base 40 to facilitate electrical connection to the powersource. As a'specific example, the lamp 10 as shown is designed tooperate with the power input of 500 to 700 watts.

In the alternative embodiment 42, as shown in FIG. 2, the arc tube orenvelope 44 is a high density sintered polycrystalline alumina bodywhich has alumina end caps 46 sealed thereto. The electrodes 48 areoperatively positioned proximate the envelope ends. At the ends of thearc tube 44 there are provided exhaust and fill tubulations 50 whichalso serve the function of supporting the electrodes 48. In accordancewith the present invention, the mercury 16, at least one of praseodymiumiodide, neodymium iodide and cerium iodide 18 and also cesium iodide 20are included within the are tube 44 in predetermined amount and halogento rare-earth metal ratio as specified for the previous embodiment,along with the small charge of inert lonizable starting gas. To completethis embodiment as an operative device, the arc tube 44 would normallybe included within an outer envelope as in the embodiment shown in FIG.1, and the general construction of such a device is well known.

As outlined hereinbefore, the essential constituents which comprise thedischarge-sustaining filling are the small charge of inert ionizablestarting gas, mercury as required to provide an operating vapor pressureof mercury per se of from 3 to 15 atmospheres, at least one ofpraseodymium halide, neodymium halide and cerium halide, excluding thefluoride, in predetermined amount and halogen to rare-earth metal ratioand cesium halide, excluding the fluoride, in predetermined amount, witha specified relative molar ratio of these rare-earth metal halides tocesium halide. The functioning of the mercury is to provide adequatevoltage drop or loading between the electrodes and in conjunction withthe cesium halide, the relatively high mercury vapor pressure and cesiumhalide function to lower the minimum envelope temperature or so-calledcoldspot temperature at which optimum lamp operating efficiency isobtained. The specified rare-earth metal halides function to produce anextremely efficient discharge, and cerium halide is preferred. As aspecific example, for an arc tube which encloses a volume ofapproximately 20 cubic centimeters and an electrode spacing of 7centimeters, mercury included in amount of approximately 200 mg. willprovide a mercury operating pressure of approximately 8 atmospheres.Along with the mercury filling are included cerium diiodide in amount of10 milligrams mixed with cerium triiodide in amount of 10 milligrams,and cesium iodide (C51) in amount of 10 milligrams. Alternatively, asimilar mixture of praseodymium diiodide and triiodide in total amountof 20 milligrams or a similar mixture of neodymium diiodide andtriiodide in total amount of 20 milligrams can be used to replace thecerium iodide in the foregoing example. These rare-earth metal iodidescan be mixed, if desired.

In order to improve the color appearance and color rendering propertieswhich are obtained from a discharge-sustaining filling as describedhereinbefore, it is highly desirable to include one or more of thefollowing halides, excluding the fluorides: sodium halide, dysprosiumhalide and/or samarium halide. The dysprosium halide and saramium halideare preferably dosed as the dihalide. The sodium halide providesradiations in the yellow-orange region of the visible spectrum,dysprosium halide provides a red and blue emission and samarium halideprovides a bluish-white emission. The supplemental rare-earthhalides'may be regarded as a partial substitute for the praseodymium,neodymium and/or cerium halides, although these supplemental halidesdesirably are present in total amount of at least 7 X 10* and preferablyat least 1.4 X 10 gram mol/cm of electrode spacing. The sodium halidemay be regarded as a partial substitute for the cesium halide, althoughthe cesium halide should always be present in amount of at least 3.5 X10' gram mol/cm of electrode spacing. The sodium halide desirably ispresent in amount ofat least 2.5 X 10 and preferably at least 3.5 X 10gram mol/cm of electrode spacing. Preferably, the supplementalrare-earth halides, as outlined hereinbefore, namely dysprosium halideand samarium halide, are present in gram mol amount which does notsubstantially differ from the gram mol amounts of the praseodymium,neodymium and/or cerium halides and the mol ratio of supplementalrare-earth halide to these required halides should not exceed 5/1.Preferably the sodium halide is present in gram mol amount which doesnot substantially differ from the gram mol amount of the cesium halideand the mol ratio of sodium halide to cesium halide should not begreater than 5/1.

As stated hereinbefore, in order to achieve a usable minimum envelopetemperature, the pressure of the mercury vapor should be present in suchan amount that if used alone as the discharge-sustaining filling, themercury vapor pressure in an operating device would be from 3 toatomospheres. For an arc tube having a volume of approximately cc asdescribed in the preferred embodiment, the mercury dosing or loadingshould be from 75 milligrams to 375 milligrams. Preferably, the mercuryvapor pressure during operation of the device should be from 4atmospheres to 10 atmospheres which will require a mercury dose of from100 mgs. to 250 mgs. (5 mgs/cc to 12.5 mgs/cc) for the preferredembodiment as described hereinbefore.

As disclosed in copending application Ser. No. 243,711 filed Apr. 13,1972, the rare-earth metal halide provides a very efficient dischargeand the cesium halide plus the high mercury loading permit a highefficiency to be' obtained with a relatively low minimum envelopetemperature. This permits a practical lamp to be fabricated whichoperates with excellent efficiency and a good color appearance. Themechanism by which this is achieved is not completely understood, but itappears that the increased mercury pressure in combinationwith thecesium halide increases the amount of the desired radiating species inthe arc, and provides a more diffuse discharge mode which has a lower,more nearly optimum arc temperature. The resulting broader arc has lesstendency to bow during operation and even when it does, this broader arcis less destructive to the envelope becuase it has a lower temperatureand is less concentrated.

Since the normal stablevalence state for cerium reacted with halogen isplus three, it is assumed that the cerium iodide, as vaporized in thearc tube, is in the trivalent state. Nevertheless, when the atom ratioof halogen to metal in the closed material is from about- 1.8:1 to about27:1, in accordance with the present invention, the resulting dischargeis even more, diffuse and more stable. As a specific example, if ceriumis dosed as the triiodide in the foregoing example, the dischargeoccupies a cross-sectional area, at the midportion of the arc tube, ofabout 6.5 mm diameter, as viewed through a filter. When the cerium isdosed in such manner that the atom ratio of halogen to metal is fromabout 1.8:1 to about 2.7:1, the resulting discharge is even more diffuseand occupies a cross-sectional area of about 13 mm diameter, as viewedat the midportion of the are tube. ln addition, the resulting discharge,when stabilized exhibits a positive resistance characteristic and nearunity power factor. This simplifies ballasting and can permit normaloperation of the device without the usual inductive current-limitingballast. An otherwise identical lamp, but dosed with the triiodide, willexhibit a negative resistance characteristic and a power factor of lessthan unity. While the foregoing results are reported for cerium halide,equivalent results tures'thereof with the cerium halide.

Other additional supplemental discharge-sustaining fillings can be used,if desired, further to modify the discharge. Examples of such additionalhalides, excluding the fluoride of course, are holmium halide, scandiumhalide, gadolinium halide, and indium halide. Also eu- .halide should beless than aboutS/l and the molar ratio of the indium halide to theindicated rare-earth metal halide should be less than about 1} 1. As amatter of of practicability, any of these additives should be present inamount of at least about 2 X '10" and preferably about 1.4 X 10 grammol/cm of spacing between the arc tube electrodes, although lesseramounts can be used if desired. Preferably, the supplemental additivesare generally used in gram mol amounts not substantially different fromthe gram mol amounts of praseodymium, neodymium and/or cerium halides,except for the sodium halide supplemental additive which desirably isused in gram mol amount not substantially different from the gram molamount of cesium halide.

While the foregoing examples have considered the iodides, it should beunderstood that the bromides or the chlorides. in equivalent gram'molamount can be substituted therefor. Alternatively, any mixture ofiodide, bromide and/or chloride can be utilized if desired.

A particularly attractive combination of dischargesustaining fillings isa combination of cerium trihalide, cerium dihalide, sodium halide,samarium as the dihalide, and cesium halide. As a specific example, foran arc tube having an electrode spacing of 7 cm. and enclosing avolumeof 20 cc., the arc tube is dosed with 200 mg. mercury, 4 mg.cerium diiodide, 4 mg. cerium triiodide,-5 mg. sodium iodide, 15 mg.samarium diiodide and 5 mg. cesium iodide. Argon starting gas is used ata pres'sure of 2O torrs. Such an embodiment opcrates with an efficiencyof lumens per watt when operated with a minimum envelope temperature of560C. The foregoing so-called dosing will provide a mercury vaporpressure of about 8 atmospheres, the cerium iodide and cesium iodide arepresent in amount of about 2 to 3 X 10 gram mol/cm of electrode spacing,and the samarium iodide and sodium iodide are present in amount of about5 X 10 gram mol/cm of electrode spacing. The discharge is stable anddiffuse and approximates a white color and the color rendition ofilluminated objects is good.

The total amount of iodide dosing in the foregoing specific lampcombination is 33 mgs. It has been found that this total iodide dosingcan be substantially reduced without materially affecting theperformance of the lamp. As an example, for the foregoing specific lamp,the total amount of iodide dosing can be reduced to 10 mgs. or evenless, while still maintaining the total mercury closing at about 200mgs. For best perform ance, however, the relative gram mol amounts ofthe iodide additives should be maintained at about the same relativemolar ratio. More specifically, the relative amounts of the indicatediodides are preferably maintained in about the following relative grammol proportions: total cerium iodide, 2 to 3; cesium iodide, 2 to 3.samarium iodide, 4 to 6.

What we claim is:

1. An arc-discharge device comprising a sealed elongatedlight-transmitting envelope which encloses a predetermined volume,electrical lead-in conductors sealed through said envelope andelectrically connected to electrodes which are operatively spaced aparta predetermined distance within said envelope, a discharge-sustainingfilling enclosed by said envelope and having the following as essentialconstituents: a small charge of inert ionizable starting gas; mercury inpredetermined amount as required, when fully vaporized as the soledischarge-sustaining constituent, to provide an operating mercury-vaporpressure of from 3 to 15 atmospheres as calculated on the basis of anaverage mercury vapor temperature of 2000K, at least one of'praseodymiumhalide, neodymium halide and cerium halide, excluding the fluoride, intotal amount of from 2 X 10' to 2.5 X 10 gm mol/cm of spacing betweensaid electrodes; cesium monohalide, excluding the fluoride, in amount offrom 3.5 X 19 to 2.5 X 1Q? gm mol/cm of spacing between said electrodes;the molar ratio of said praseodymium halide, neodymium halide and ceriumhalide to said cesium halide being from 4/ l to 1/25; and the ratio oftotal gram-atoms of halogen to total gram-atoms of metal in saidpraseodymium halide, said neodymium halide and said cerium halide beingfrom about 118:] to about 2.7:1.

2. The arc-discharge device as specified in claim 1, wherein saidpraseodymium halide, neodymium halide and cerium halide are present intotal amount of from .51. 93 am mollsmsztsats nsbs; tween saidelectrodes; and said cesium halide is present in amount of from 3.5 Xl0- to 5.4 X 10 gm mol/cm of spacing between said electrodes.

3. The arc-discharge device as specified in claim 2, wherein saidmercury is present in predetermined amount as required when fullyvaporized to produce an operating mercury vapor pressure of from about 4to about 10 atmospheres.

4. The arc-discharge device as specified in claim 3, wherein saidhalides are the iodides.

5. The arc-discharge device as specified in claim 3, wherein saidmercury is present in amount of from 5 to 12.5 mgs/cc of saidpredetermined volume of said envelope.

6. The arc-discharge device as specified in claim 1, wherein there isalso included within said envelope as supplemental discharge-sustainingfilling at least one halide, excluding the fluoride, of sodium halide,dysprosium dihalide, and samarium dihalide, the molar ratio of saiddysprosium halide and said samarium halide to said praseodymium andneodymium and cerium halide being less than about 5/ 1 and the molarratio of said sodium halide to said cesium halide being less than about5/1.

7. The arc-discharge device as specified in claim 6, wherein saiddysprosium halide and said samarium halide are present in amount of atleast 7 X 10" gm mol/cm of spacing between said electrodes, and saidsodium halide is present in amount of at least 2.5 X 10' gm mol/cm ofspacing between said electrodes.

8. The arc-discharge device as specified in claim 6, wherein saidhalides are the iodides.

9. The arc-discharge device as specified in claim 8, wherein saidmercury vapor pressure is about 8 atmospheres, and said cerium iodide,said cesium iodide, said samarium iodide and said sodium iodide arepresent in about the following proportions, as expressed in terms ofrelative gram mol amounts: cerium iodide, 2 to 3; cesium iodide, 2 to 3;samarium iodide, 4 to 6; and sodium iodide, 4 to 6.

9. The arc-discharge device as specified in claim 8, wherein saidmercury vapor pressure is about 8 atmospheres, and said cerium iodide,said cesium iodide, said samarium iodide and said sodium iodide arepresent in about the following proportions, as expressed in terms ofrelative gram mol amounts: cerium iodide, 2 to 3; cesium iodide, 2 to 3;samarium iodide, 4 to 6; and sodium iodide, 4 t o 6. w W

